Water-Stable\nCathode for High Rate Na-Ion Batteries

Abstract

Most\nof sodium-layered oxide cathodes are unstable under moisture conditions.\nAs a unique water-stable cathode, Na_2/3Ni_1/3Mn_2/3O₂ (NNM) usually becomes vulnerable to\nwater molecules after element substitution treatment to suppress the\nNa⁺ vacancy ordering arrangement, which causes limited\nNa⁺ diffusion kinetics. Herein, we show that these issues\ncan be addressed simultaneously by rational designing the transition-metal\n(TM) layer to achieve both water-stable and Na⁺ vacancy\ndisordering structures. Density functional theory calculations reveal\nthat the water-stability of the layered oxide cathode can be correlated\nto the surface adsorption energy of H₂O molecules. In the\nTM layer, the Co/Mn and Fe/Mn units exhibit a much lower adsorption\nenergy than that of the Li/Mn unit, and hence the H₂O molecule\nprefers to be absorbed on Co/Mn and Fe/Mn units rather than Li/Mn.\nMoreover, the Li/Mn unit in the TM layer can suppress the Na⁺ vacancy ordering structure in NNM to improve the Na⁺ diffusion\nkinetics. As a consequence, the well-designed Na_2/3Li_1/9Ni_5/18Mn_2/3O₂ cathode can\nnot only maintain its original crystal structure and electrochemical\nproperty after water soaking treatment but also exhibit high rate\ncapability (78% capacity retention at 20 C) and excellent cycling\nstability (87% capacity retention after 1000 cycles).